"A gender-expansive person’s preferences and self-expression may fall outside commonly understood gender norms within their own culture; or they may be aligned with them even as one’s internal gender identity doesn’t align with the sex assigned at birth."

From the way I understand it, gender identity = what one 'feels like', although how can one feel non-binary is up to anyone.

Furthermore, when they say 'gender expressions create gender', what they mean is gender as we understand it, in other words, what we associate with men/women.

"Non-binary people may wish to transition so that their gender expression more closely reflects their internal identity. Many non-binary people wish to appear androgynous and adopt unisex names, gender-neutral titles such as Mx. and/or gender-neutral pronouns, but others prefer to express themselves in ways which are traditionally seen as masculine or feminine or to mix aspects of the two."

And:

"E might be seen by others as male, Jane wrote, now that her hair was so short and her clothing so androgynous. She would probably use “both male and female bathrooms depending on what situation feels safest,” Jane informed the coach, and “will need to tell you when she is going to the restroom and what gender she plans on using.”"

http://archive.fo/peJQ3

Ultimately, I feel like this points it out quite good: "I hate how the binary world pressures me into a binary state and invalidates my non-binary gender identity. I feel happy in the grey zone where I'm neither a man or a woman."